The present invention relates generally to keyless entry systems. More particularly, the invention relates to a method for automatically resynchronizing the transmitter/receiver pair when synchronization is lost due to momentary power failure or a low battery condition, for example.
Rolling code authentication is a common form of vehicle entry security. In such a system, a transmitter is provided in the form of a key fob and a receiver is positioned in the vehicle where it is able to receive encoded transmission from the key fob transmitter. Rolling code authentication can be performed by employing a simple linear counter which advances with each key fob command. The receiver in the vehicle is configured to always expect an increasing value and therefore it disallows repeating counter values. Thus to be in sync the transmitter counter should never fall behind the count of the receiver, nor should the transmitter counter be permitted to get too far ahead of the receiver count. More complex authentication using linear shift feedback register (LFSR) technology is also used as a more secure technique for vehicle entry security.
For a number of reasons, a rolling code authentication system can occasionally fall out of synchronization when the counter values of the transmitter are less than that of the receiver or when the transmitter counter values are greater than those of the receiver by a predetermined number. One reason for losing synchronization is corrupted volatile memory, which can be due to momentary loss of battery power or low battery voltage.
One way to ensure against loss of synchronization is to outfit the transmitter with a nonvolatile memory such as an EEPROM which can be used to store the rolling values so they will not be lost. Being nonvolatile, the EEPROM will not lose synchronization due to a power interruption (e.g. loose battery connection or battery failure). The EEPROM protects the integrity of the counters when the internal RAM is powered-off.
However, EEPROM devices are comparatively expensive and it would be desirable to eliminate them from the rolling code authentication circuitry. This presents a problem, since without nonvolatile memory, a system would have to rely on RAM (volatile memory) to store counter values. The need to rely on RAM increases the possibility of corrupted counter values, since even temporary loss of power through a loose battery connection or loss of battery charge would break synchronization.
The present invention addresses the synchronization problem by providing a system which automatically resynchronizes the transmitter/receiver pair when corrupted memory is detected. Ordinarily the user is not even aware resynchronization is being performed. This is accomplished by calculating and storing a first checksum indicative of the state of the transmitter at a first instance in time. Thereafter, the communications sequence is initiated between transmitter and receiver and the initiation of this sequence automatically causes a second checksum to be calculated. The second checksum is indicative of the state of the transmitter at that second instance in time. The checksums are compared and if the checksums are different, the resynchronization sequence is automatically initiated.
For a more complete understanding of the invention, its objects and advantages, reference may be made to the following specification and to the accompanying drawings.